1. Field of the Invention
The present invention relates to a process for producing 1,3-bis(dicarboxyphenyl)disiloxane derivatives or dianhydrides thereof which are useful as raw materials of polyimide resins, as hardeners for epoxy resins, and as others.
2. Description of the Prior Art
Of the 1,3-bis(dicarboxyphenyl)disiloxane derivatives represented by the general formula [V], ##STR7## wherein, Rs and R's independently of one another denote methyl or phenyl, a compound having methyl groups as Rs and R's and containing carboxy groups at the 3- and 4-positions of each phenyl group, viz. 1,3-bis(3,4-dicarboxyphenyl)-1,1,3,3-tetramethyldisiloxane represented by the formula [VII], ##STR8## can be synthesized by a known process described in J. Org. Chem., Vol. 38, p. 4271 (1973). This synthetic process is based on the following reaction scheme [VIII]: ##STR9##
That is, the process comprises reacting 4-bromo-o-xylene with n-butyllithium, reacting the resulting lithium compound with dichlorodimethylsilane, reacting the resulting dimethyl (3,4-dimethylphenyl)-chlorosilane with thiophene, reacting the resulting 2-(3,4-dimethylphenyldimethylsilyl) thiophene with dimethyl (3,4-dimethylphenyl)chlorosilane, oxidizing the resulting 2,5-bis(3,4-dimethylphenyldimethylsilyl)-thiophene with potassium permanganate, and acidifying the resulting aqueous solution of potassium carboxylate to precipitate and recover the intended tetracarboxylic acid of formula [VII]. Said paper also describes the method of treating with acetic anhydride during the reaction and reports that the melting point of the thus obtained dianhydride represented by the formula [XII], ##STR10## was found to be 137.degree.-138.degree. C.
A process based on the reaction scheme [IX]: ##STR11## is described in "polyimides", Vol. 1, p. 51, edited by K. L. Mittal, published by Plenum Press. That is, the process comprises hydrolyzing dichlorodimethylsilane, reacting the resulting 1,3-dichlorotetramethyldisiloxane with a lithium compound which is the reaction product of 4-bromo-o-xylene and n-butyllithium, oxidizing the resulting 1,3-bis(3,4-dimethylphenyl)-1,1,3,3-tetramethyldisiloxane with potassium permanganate, and acidifying the resulting aqueous solution of potassium tetracarboxylate to precipitate and recover the intended tetracarboxylic acid of formula [VII]. The "Polyimides", Vol. 1 further reports that recrystallization from n-hexane gave a dianhydride of formula [XII]having a melting point range of 156.degree.-159.degree. C.
Japanese Patent Application Laid-Open No. 83191/86 describes another process based on the following reaction scheme [X]: ##STR12##
That is, the process comprises reacting 1,2-dimethoxytetramethyldisilane with N-n-butyl-4-chlorophthalimide, hydrolyzing the resulting 4-(dimethylmethoxysilyl)-N-n-butylphthalimide, followed by alkali-hydrolysis of the resulting bisphthalimide compound of disiloxane to give the product tetracarboxylic acid.
All the above described prior art processes, however, have the disadvantages of many reaction steps required and low yields of intermediates and one of these processes employs a special compound as a raw material.
That is, in the process described in J. Org. Chem., based on reaction scheme [VIII], the yield of dimethyl(3,4-dimethylphenyl)chlorosilane, which is an intermediate, is as low as 56%, while the yield of the end product of formula [VII] is uncertain (the yield is not described). In this process, the phthalic anhydride groups of the compound of formula [XII] is derived from 4-bromo-o-xylene. However, industrial synthetic processes provide 4-bromo-o-xylene contaminated with about 25% by weight of 3-bromo-o-xylene, such mixtures only being available. In consequence, when the synthesis of the dianhydride of formula [XII] is tried by using such a mixture of 4-bromo-o-xylene and 3-bromo-o-xylene, the silicon-containing tetracarboxylic dianhydride only obtained is a mixture composed of isomers represented by the general formula [XI], ##STR13##
When this isomeric mixture is used as the raw material of a polyimide resin, the resulting resin has a low degree of polymerization and is inferior in heat resistance. In order to solve this problem, the dianhydride of formula [XII]should be separated from the isomeric mixture and purified before use as the raw material of a polyimide.
The present inventors also tried the separation and purification by using each of acetic anhydride and n-hexane as stated above, finding the following problems.
The product yield from the purification with acetic anhydride was very low since the isomeric mixture is highly soluble in acetic anhydride.
The recrystallization from n-hexane was found to need a vast volume of n-hexane because the dianhydride of formula [XII] is scarcely soluble in n-hexane.
According to the process based on the reaction scheme [IX], described in "Polyimides" edited by K. L. Mittal, 1,3-dichlorotetramethyldisiloxane is obtained as an intermediate by hydrolysis of dichlorodimethylsilane that is a fundamental compound in silicon chemistry. In this case, however, polysiloxane compounds such as 1,5-dichlorohexamethyltrisiloxane are produced incidentally and the yield of 1,3-dichlorotetramethyldisiloxane is only 22%. Moreover, 1,3-bis(3,4-dimethylphenyl)-1,1,3,3-tetramethyldisiloxane is obtained but in a low yield of 36% by the double cross coupling of the 1,3-dichlorotetramethyldisiloxane with a lithium compound. Furthermore, since 4-bromo-o-xylene is also used in this process, only a mixture of isomers represented by general formula [XI] can be obtained as in the process described in J. Org. Chem., Vol. 38.
According to the process based on the reaction scheme [X], described in Japanese Patent Application Laid-Open No. 83191/86, 4-(dimethylmethoxysilyl)-N-n-butylphthalimide is obtained by reacting N-n-butyl-4-chlorophthalimide with 1,2-dimethoxytetramethyldisilane under toluene reflux in the presence of a palladium catalyst, but the disilane compound used in this case is a quite special compound expensive and dangerous in that spontaneous ignition thereof takes place in the air. This compound is used as much as four times the theoretical amount for the reaction. Although the yield of intermediate 4-(dimethylmethoxysilyl)-N-n-butylphthalimide is reported to be 83% on the basis of N-n-butyl-4-chlorophthalimide, no description is given about the yield of the bisphthalimide of disiloxane produced in the next step and only the possibility of production is set forth about the compound of formula [VII].
As described above, no industrial process has been established for producing the silicon-containing tetracarboxylic acid of formula [VII], those of general formula [V], or the dianhydride of formula [XII].